Your car manufacturer's maintenance schedule will tell you how often the transmission fluid should be changed. Be sure to factor in how often and how hard you drive your car. Experts define severe use on a car as driving it more than 50 percent of the time in heavy city traffic, with temperatures above 90 degrees F (32.2 degrees C). If this sounds like you, the recommended interval for changing the transmission fluid is every 15,000 miles (24,140 kilometers).